Conventionally, a load sensor, especially a load cell, has employed a cantilever construction such that, as shown in FIGS. 14 and 15, one end of an arm 50 is fixed, and a bearing plate 53 is provided at the other end thereof to receive a load. In such a load cell, strain gauges 54 that are deformed by an applied load are directly affixed to the arm 50, by which the applied load is measured from the strain on the arm 50. A hole 51 is formed on the inside of the arm 50, and thin wall sections 52 having a small thickness are provided at two places in each of an upper part and a lower part. The strain gauges 54 are affixed to these thin wall sections 52 to detect the strain of the thin wall sections 52. In such a load cell, as shown in FIG. 14, moderate strain are produced on the thin wall sections 52 by deforming the arm 50 in a state in which the arm 50 is kept in a parallelogrammatic shape, by which the applied load can be measured exactly. A load cell having the above-described configuration is generally called a Roverval mechanism.
In this load cell of Roverval mechanism, as shown in FIG. 15, if a load is applied, together with the intended load, onto the bearing plate 53 at positions P1 and P2 shifting in the right and left direction perpendicular to the axial direction in which the arm 50 extends, an error occurs on the detected load. To restrain this error, four-corner adjustment such that corners of the thin wall sections 52 are scraped on the trial and error basis while the load application portion is changed has been made.
However, as shown in FIG. 15, if load noise F1 or F2 from the direction unnecessary for load measurement is applied at the same time that the intended load is applied, the error cannot be restrained, and thus the measurement results are influenced adversely.
In recent years, in the field of automobile, to further improve safety, a technique has increasingly been used in which physical structures including body weight of a passenger seated on a seat are judged, and based on the judgment result, the operation of safety device such as an air bag is controlled comprehensively. In such a technique, it is a vertical load that is necessary for measuring the body weight of passenger. A horizontal load produced at the time of acceleration, deceleration, etc. of automobile exerts an adverse influence on the measurement result. The horizontal load includes, for example, vibrations produced by the irregularities of road surface, and various kinds of accelerations produced by starting, braking, or steering wheel operation. Therefore, a load sensor capable of removing loads acting from directions other than the vertical direction is demanded.
In this respect, to avoid the above-described problem, Patent Document has disclosed an invention relating to a load cell in which a portion that is displaced by the application of force and a portion that is deformed corresponding to the displacement of the aforementioned portion are provided separately, and these portions are integrated. According to this load cell, the displaced portion and the deformed portion are connectingly supported by a connecting shaft, by which an adverse influence exerted by an unbalanced load is restrained as far as possible.
The use of the load cell described in this Patent Document for an automobile can be considered as one plan.
Patent Document 1: Japanese Patent Laid-Open No. 2003-166885